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WATER RESOURCES RESEARCH, VOL. ???, XXXX, DOI:10.1002/,
Distributed Temperature Sensing as a down-hole tool1
in hydrogeology2
V.F. Bense1, T. Read
2, O. Bour
3, T. Le Borgne
3, T. Coleman
4, S. Krause
5,
A. Chalari4, M. Mondanos
4, F. Ciocca
4, and J.S. Selker
6
1Department of Environmental Sciences,
Wageningen University, The Netherlands.
2School of Environmental Sciences,
University of East Anglia, Norwich
Research Park, Norwich, NR4 7TJ, UK.
3Geosciences Rennes, University of
Rennes 1, Rennes, France.
4Silixa Ltd, Elstree, Hertforsdshire, WD6
3SN, UK.
5School of Geography, Earth and
Environmental Sciences, University of
Birmingham, Birmingham, B15 2TT, UK.
6Biological and Ecological Engineering,
Oregon State University, Corvallis, Oregon,
USA.
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Abstract. Distributed Temperature Sensing (DTS) technology enables3
down-hole temperature monitoring to study hydrogeological processes at un-4
precedentedly high frequency and spatial resolution. DTS has been widely5
applied in passive mode in site investigations of groundwater flow, in-well6
flow, and subsurface thermal property estimation. However, recent years have7
seen the further development of the use of DTS in an active mode (A-DTS)8
for which heat sources are deployed. A suite of recent studies using A-DTS9
down-hole in hydrogeological investigations illustrate the wide range of dif-10
ferent approaches and creativity in designing methodologies. The purpose11
of this review is to outline and discuss the various applications and limita-12
tions of DTS in down-hole investigations for hydrogeological conditions and13
aquifer geological properties. To this end, we first review examples where pas-14
sive DTS has been used to study hydrogeology via down-hole applications.15
Secondly, we discuss and categorize current A-DTS borehole methods into16
three types. These are thermal advection tests, hybrid cable flow logging, and17
heat pulse tests. We explore the various options with regards to cable instal-18
lation, heating approach, duration, and spatial extent in order to improve19
their applicability in a range of settings. These determine the extent to which20
each method is sensitive to thermal properties, vertical in well flow, or nat-21
ural gradient flow. Our review confirms that the application of DTS has sig-22
nificant advantages over discrete point temperature measurements, partic-23
ularly in deep wells, and highlights the potential for further method devel-24
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opments in conjunction with other emerging fiber optic based sensors such25
as Distributed Acoustic Sensing.26
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1. Introduction
Heat is widely recognised as an excellent tracer for a range of hydrogeological processes27
that can be inferred from temperature-depth measurements in boreholes [e.g., Saar , 2011].28
The advantage of using heat over other tracers such as geochemical tracers, is that heat29
is ubiquitous, and easily and economically measured in-situ with a variety of probes30
and loggers with resolutions down to 10−3 ◦C [Pehme et al., 2013]. Near-surface (upper31
several m) temperature measurements in either shallow piezometers or by pushing probes32
into the ground [e.g., Bense and Kooi , 2004; Conant , 2004; Schuetz and Weiler , 2011]33
can be used to identify and quantify rates of groundwater-surface water interactions by34
exploiting the effect that groundwater upwelling or downwelling affect the propagation of35
the seasonal/diurnal temperature wave into the subsurface [Taniguchi , 1993].36
At depths greater than ∼20 meters seasonal surface temperature fluctuations can usu-37
ally no longer be detected. Here the geothermal gradient usually dominates temperature-38
depth profiles which is a function of both the local geothermal heat flux and thermal39
conductivity of the formation, primarily governed by Fourier’s Law of heat conduction.40
Where a temperature-depth profile departs from this expected behaviour, it is often in-41
dicative of groundwater flow which drives the advection of heat in addition to conduc-42
tion. Temperature-depth profiles can become convex or concave for upward or downward43
groundwater flow respectively [e.g., Bredehoeft and Papadopulos , 1965] whilst localized44
horizontal groundwater flow through fractures or faults often results in localised abrupt45
temperature changes that depart from the geothermal gradient [Ge, 1998; Bense et al.,46
2008].47
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The use of Raman-scatter based optical-fibre sensors to infer a spatial distribution of48
temperature along the fibre hosted in an enclosure (e.g., cable tubing), commonly known49
as Distributed Temperature Sensing (DTS) has seen a steep rise in popularity in studies of50
groundwater-surface water interactions [e.g., Selker et al., 2006; Briggs et al., 2012a; Hare51
et al., 2015] in which it forms an attractive alternative for conventional point temperature52
measurements . To a lesser extent DTS has been deployed to evaluate hydrogeological53
conditions at depths below the seasonal zone (e.g., > 20 m), but these can only be accessed54
via boreholes (up to several km deep) or, in unconsolidated sediments, via direct-push55
methods (up to ∼80 m). The latter application will be the focus of this review.56
This paper focusses on discussing the current state and range of options and techniques57
involving DTS technology in down-hole settings to investigate groundwater hydrological58
conditions. After summarising the principles of DTS with special attention to the rela-59
tionship between resolution in space and time, we review the feasibility and limitations60
of DTS technology for groundwater studies in down-hole applications.61
2. Principles of Distributed Temperature Sensing
DTS uses the Raman backscatter characteristics of light emitted following a laser pulse62
into a fiber optic cable to determine the distributed temperature along fiber as long as 3563
km. Raman backscatter is generated by the inelastic interaction of the incident light with64
molecular vibrations in the fibre that causes the energy level of the emitted photons to be65
shifted up or down relative to the injected light [Rogers , 1999] (Figure 1). A measurement66
of the ratio of the intensity of photons returning at specific higher (anti-Stokes) and specific67
lower wavelengths (Stokes) in time allows for the calculation of the temperature along68
a fibre allows the computation of temperature as a function of distance [Farahani and69
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Gogolla, 1999; Hausner et al., 2011]. In most hydrological applications DTS control units70
are used that determine distance via the transit time of backscattered light by exchanging71
time for distance, under the same principles as an optical time domain reflectometer72
(OTDR) [Dakin et al., 1985]. In an alternative implementaiton, DTS systems can also be73
based upon optical frequency-domain reflectometry (OFDR) [Bolognini and Hartog , 2013].74
Both OTDR and OFDR techniques use the fiber itself as a continuous sensing element75
that is interrogated over a certain distance of cable. Alternatively, at point locations76
the fiber Bragg gratings (FBG) may be etched into the fiber which allows to obtain a77
temperature at point locations [Guan et al., 2013]. To discuss the specifics and potential78
dis- and advantages of these DTS techniques (OTDR/OFDR-based, or using FBGs), and79
other possible approaches (e.g., the use of different wavelengths for the emitted light) goes80
beyond the scope of this review.81
2.1. Temperature calibration and monitoring modes
After sending out high-frequency laser pulses, a DTS instrument analyses the Stokes82
and anti-Stokes intensities integrated over user specified time and spatial intervals along83
the cable. From these data the average temperature for each increment of length along84
a fiber optic cable is calculated using three calibration parameters. Values for these85
calibration parameters are typically obtained by use of an instrument-internal reference86
coil of fiber in combination with internal and external temperature probes attached to the87
DTS unit. However, light loss (e.g., from fibre damage or where it has been spliced) at any88
location along the path can result in abrupt offsets in computed temperature along the89
cable which values easily exceed ±1 K. Much more accurate temperatures (on the order90
of 0.02 C) can be obtained through use of external reference temperature baths [Tyler91
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et al., 2009], and post-processing of the observed Stokes and anti-Stokes data to calculate92
temperature values [Hausner et al., 2011; Van De Giesen et al., 2012]. These reference93
baths must create homogeneous temperature conditions, which need to be constant over94
the measurement integration time, in which lengths of fiber optic cable of for example at95
least 10-times the sampling interval, and fully external reference temperature loggers are96
installed [Hausner et al., 2011; Van De Giesen et al., 2012].97
Ideally, calibration is based upon at least three externally measured temperatures taken98
along each section of fiber (i.e., between locations of splices or connectors), two of which99
must be separated in space. When there are three locations with known temperatures100
(e.g. water baths in which temperatures are constantly homogenised), but a splice or101
damage exists at a location in between, it is possible to correct the ratio of the Stokes and102
anti-Stokes intensities to account for the differential attenuation occurring at this point103
[Hausner and Kobs , 2016]. The two general configuration options for the optical path104
of the fiber are known as single-ended and double-ended configurations, and cables may105
be produced as simplex or duplex (i.e., with either one or two fibers in the same cable)106
each of which presents specific options for calibration. Generally speaking, duplexed and107
double-ended operations allow for more accurate calibration since multiple measurements108
are then taken at each location along the cable. In borehole deployments any of these109
options may be appropriate and are outlined briefly here.110
In a conventional single-ended set up, a single fiber is coupled to the base unit and111
terminated at the other end. This cable can be looped back in and out of a borehole [Read112
et al., 2013] in which case in principle the temperature-depth profile should be symmetrical113
around the downhole turn-around point. If the loop constructed within a single cable by114
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connecting two fibers at the remote end, this would be called a duplexed installation. In115
either case, the looped set-up and the consequent symmetry of the measurement can be116
used to depth locate the observed temperatures (Figure 1). Duplexed cables do present117
certain potential issues. First, the splice at the end of the cable must be protected, which118
in the case of deep boreholes, can require a high-pressure enclosure. Further, the data119
after the splice need to be adjusted for potential differential loss of the return signals at120
the fusion splice [Hausner et al., 2011]. Additionally, such splices can generate spatially121
distributed defects in temperature measurement, which can be challenging to detect and122
correct for [Arnon et al., 2014a].123
Double-ended installations are those where the optical path of the fiber both begins124
and ends at the DTS, so that light might be passed both directions along the fiber.125
Measurements are taken alternately from each instrument channel, which are referred126
to as the forward and reverse directions [Van De Giesen et al., 2012]. In this way it is127
possible to compute calibration parameters for each section of the fiber to adjustment128
for non-uniform differential attenuation along the cable occurring at bends, connectors,129
splices, and where there has been fiber degradation (e.g., hydrogen ingress). The key130
point here is that the calibration strategy is dictated by the fiber layout, and must be131
considered in detail as part of the experimental design.132
2.2. DTS performance metrics
The key metrics of DTS system performance are the accuracy and precision of reported133
temperature, at the specified spatial resolution and integration time. The uncertainty in134
temperature is typically characterized by the standard deviation or Root Mean Square135
Error (RMSE) of the reported temperature in comparison to the true temperature. The136
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RMSE in temperature normally scales with the square root of the product of the inte-137
gration time and the length of cable over which the temperature is being reported. This138
reflects the central limit theorem applied to the number of photons employed in the inten-139
sity estimates. The RMSE is also a function of distance along the cable due to Beer’s Law140
attenuation of light with travel distance from the base unit, with data further from the141
instrument having lesser accuracy. The relationship between RMSE and distance is more142
complicated in the case of double ended measurements, wherein reported temperatures143
are computed using light travelling in both directions along the cable. For double-ended144
installations the lowest RMSE occurs at the mid-point of the cable equidistant from the145
base unit along each channel.146
DTS units can be set to report back temperatures at a range of sampling intervals,147
wherein the the spatial resolution is always at least twice than this value by the Nyquist148
theorem [Selker et al., 2014]. The limiting spatial resolution of a DTS is determined at149
a step change in temperature along a cable, and is defined as the distance between the150
points for example at 10% and 90% of the true temperature change [Tyler et al., 2009;151
Selker et al., 2014]. Putting these concepts together lead DTS manufactures to report152
measurements the less than half of the limiting spatial resolution. Each reported Stokes153
and anti-Stokes backscatter intensity, and hence the calculated temperature, at a specific154
distance is an average weighed by a Gaussian function along the sampling interval (Figure155
2). When discussing temporal resolution we mean it to be the ability to detect temperature156
change in time. The temporal resolution of an measurement configuration is limited by a157
combination of the performance of the DTS instrument and thermal inertia of the cable158
used. In groundwater systems temperatures are only slowly changing naturally at a rate159
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well within the temporal resolution of any DTS system, however in A-DTS configurations160
temperatures will be changing at a much higher rate and the temporal resolution of the161
system used will need to be considered more carefully [e.g. Read et al., 2014]. Modern162
DTS systems typically can sense temperature distributions along the entire cable every163
few seconds. Hence, cable thermal properties are commonly the most significant factor in164
thermal responsiveness. Larger diameter and more massive cables of high thermal inertia165
respond more slowly to ambient temperature changes. As long as data storage is not a166
limiting factor, it is usually advisable to set the spatial and temporal averaging to the167
highest possible frequency the instrument supports, since internal averaging applied by the168
instrument cannot be subsequently undone. Typically post-processing averaging provides169
all the advantages which would be obtained from longer collection intervals. This should170
be confirmed for each DTS base unit as we know of one manufacturer employs methods171
that violate this assumption wherein there is a penalty to selecting high temporal and172
spatial resolution. In all cases data collected at high frequency can lead to unwieldily173
data sets (e.g., from a long cables and long-duration installations). Furthermore, on some174
instruments data can be recorded faster on the instrument than it can be downloaded,175
resulting in the memory limit of the instrument eventually being reached.176
2.3. Cable selection and installation
In shallow wells it is typically possible to install a fibre-optic cable by spooling it off into177
the well to the desired maximum depth and left to monitor temperature. In deeper wells178
the feeding of the cable into the entrance of the well may need to be addressed to support179
the weight of the cable and limit bending radii. Fiber optic cables must be protected180
from damage due impact from pumps or co-located instruments. In deep wells fiber optic181
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cables are often installed in the cement behind the borehole casing where they are non-182
replaceable. So-called pump down systems are commonly used in oil and gas applications183
[Smolen and van der Spek , 2003], but not so in hydrogeology. These use a small diameter184
open tube sitting inside the borehole in which a fibre-optic cable can be led down and185
replaced if necessary after deterioration of the fibre over time for example after prolonged186
exposure to high temperatures in a geothermal heat production or oil well.187
Short-range DTS units (under 10 km) generally use multi-mode fibers (typically 50 µm188
core diameter, 125 µm cladding diameter). The added glass cross-section allows injection189
of greater light intensity, and captures a larger fraction of the backscatter compared190
to single-mode fibers, which results in higher measurement performance. Cable design191
that includes multiple fibers are advisable for down-hole applications as these provide192
redundancy in case of damage occurring to one, and allows single-ended or double ended193
measurements to be obtained by fusion splicing two fibers together at the far end of the194
cable (Figure 1).195
Every fusion splice needs physical protection. Above ground splice protection is pri-196
marily against movement of the fiber and exposure to water. Downhole splices must197
be protected against pressure using specially designed enclosures, sometimes potting the198
splices in rigid resin. The entire fiber must be protected pressure which can cause non-199
uniform differential attenuation. Fibers are often hermetically sealed into rigid (plastic200
or stainless steel, depending on required pressure rating) capillary tubes. Since the fiber201
must be completely protected from mechanical stress, the fibers path within the capillaries202
is helical to accommodate slight changes in the cable length if it is put under tension or203
experiences significant temperature changes. This results in an optical path that is about204
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0.5 percent longer than the cable. For sites that may contain organic compounds the205
cable design should include hydrogen-scavenging gel. Further protection from hydrogen206
ingression is obtained by encasing the stainless steel capillary tube in aluminum, which207
has much lower gas permeability [Reinsch et al., 2013]. Additional crush resistant armor-208
ing, tensile elements, and abrasion resistant plastic jacket may be needed. Selection of209
jacket material also must accommodate any markings needed, such as distance increments.210
In down-hole applications, cables require sufficient tensile strength to support their own211
weight over the depth of the deployment without transferring stress to the fiber.212
In borehole installations it is generally advantageous to seek the highest spatial resolu-213
tion possible. This will enhance the prospect to resolve individually, for instance, closely214
spaced fractures that carry groundwater flow. As stated above, for most DTS systems,215
should lower spatial resolution be found to be sufficient, the data may be averaged in216
post-processing with no loss in performance relative to having obtained the data at lower217
spatial resolution from the outset. Averaging of high spatial resolution data may be ap-218
propriate when carrying out heat pulse tests in settings where the geology is known to219
be relatively homogeneous of large depth intervals. In fractured aquifers, where fracture220
flow may cause localized cooling, averaging high spatial frequency data into a lower spa-221
tial frequency composite may completely average out this response. One may achieve a222
desired spatial resolution either by selecting an instrument with the desired spatial res-223
olution, or, a wrapped cable can be deployed [Hilgersom et al., 2016]. In such a cable,224
the optical fiber is wrapped around a central strength member, such that for every unit225
length of cable there is a greater length of optical fiber, so the effective spatial resolution226
is increased. Custom made solutions have been used to measure shallow temperatures227
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with depth in glaciers and lakes [Selker et al., 2006], and stream bed sediments [Briggs228
et al., 2012b; Vogt et al., 2010], while pre-wrapped cables can also can be purchased and229
have been deployed in lakes [Arnon et al., 2014a], and boreholes [Banks et al., 2014]. Data230
from wrapped cables needs careful processing. In the case of tightly wrapped fibers, there231
have been documented entrance effects in approximately the first 100 m of fiber after the232
transition from a straight cable to wrapped cable that if not corrected for, will result in233
erroneous data [Arnon et al., 2014a]. Because of the distance and time varying nature234
of the correction, Arnon et al. [2014b] developed an empirically based method for the235
processing of wrapped cable data.236
To illustrate the various spatial averaging options, it is useful to look in detail at a237
short 3 m section of a borehole at the Ploemeur research site, France, using different238
combinations of instruments and cables (Figure 2). Data were collected by instruments239
with manufacturer specified spatial resolutions of 2.0 and 0.3 m (sampling at 1.01 and240
0.12 m respectively) on standard (Figure 2 panels b and c), and wrapped cables (Brugg241
High Resolution cable, Brugg, Switzerland, Figure 2 panels d and e). An open fracture242
was encountered at a depth of 23.8 m depth, providing an outflow for groundwater which243
flowed up the borehole. In such scenario a change of the thermal gradient at the location244
of the fracture would be expected which could be used to infer the rate of inflow at the245
location of the fracture [Klepikova et al., 2014]. However, in this example the impact246
on the temperature gradient of this outflow is very subtle, but detectable, as is apparent247
from the point temperature measurements shown in Figure 2a. Data from the 2 m spatial248
resolution instrument and straight cable would give the impression of a relatively smooth249
temperature depth profile and is unable to detect the change in gradient. In this case,250
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the 2 m Gaussian spatial weighting of each temperature measurement spans the cased,251
rock, and fractured rock intervals, rendering these features indistinguishable. In contrast,252
the 0.3 m spatial resolution instrument with the straight cable (Figure 2c) allows identi-253
fication of the location and quantification of the step temperature change at the fissure,254
and would provide an improved characterization of the change in gradient over the point255
measurements (Figure 2a). Wrapping the cable to enhance the spatial resolution in com-256
bination with either low-res or high-res DTS instrument (2d and e) does not seem to result257
in a detection of the changing gradient at the location of the fracture. This is probably258
because as a result of the wrapping greater noise is introduced in the data due to the259
Beer’s law light loss along the additional optical path length, and further from light loss260
that occurs at bends. The effective spatial averaging in Figure 2d) is similar to Figure 2c),261
but for the latter reason, the temperature resolution is lower. Pairing the 0.3 m spatial262
resolution instrument with the wrapped cable gives an effective spatial resolution of 0.03263
m, which is now comparable to the spatial extent of the fractures but still the change264
in temperature gradient is hardly visible at the location of the fracture (at a depth of265
23.8 m). Concluding, in this installation only one DTS configuration was successful in266
detecting the fracture flow process of central interest in this study. This example shows267
the care required to effectively employ DTS. It is often advisable to pre-test the design of268
the field installation, choice of DTS, and cable configuration.269
3. DTS monitoring in Passive mode
DTS measurements are termed ”passive” if there is no active heating of the cable itself270
or in the DTS monitored well. In piezometers or cased boreholes, it can sometimes be271
assumed that the temperature measured in the fluid at a given depth is the same as the272
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temperature in the formation. In open or long screened boreholes, this is often not the273
case. The inflow of water with different temperature to the resident water causes small274
inflections in the temperature-depth profile and is useful to identify hydraulically active275
fractures [Drury et al., 1984]. For large inflows of distinctly different temperature, or in276
the case of warmer water entering below cooler water, the temperature signal may advect277
through the well. This temperature signal may allow the flow to be estimated using278
analytical [Drury et al., 1984], or numerical modeling approaches [Klepikova et al., 2011].279
DTS has been used to measure temperature-depth profiles in the subsurface in a variety280
of settings. Grosswig et al. [1996] monitored seasonal temperature changes in the subsur-281
face at a waste disposal site. They also detected a warm temperature signal just beneath282
the zone of seasonal fluctuation, attributed to exothermic reactions in surrounding waste283
material. DTS has also been used to assess lithological changes from passive temperature284
data in boreholes assuming that changes in temperature gradient reflect variations in ther-285
mal properties coupled to lithology [Foerster et al., 1997; Wisian et al., 1998]. Foerster286
et al. [1997], concluded that DTS was suitable for studies of terrestrial heat flow, but287
had a temperature resolution 5 to 10 times less than their conventional logging approach.288
However, we note that current instruments would be able to significantly improve on the289
latter result. Henninges et al. [2005] used a similar approach to Foerster et al. [1997]290
and Wisian et al. [1998] in an area of permafrost, and quantified thermal properties with291
depth by assuming assuming a uniform vertical heat flow density. The distinct advantage292
of using DTS in this setting is that the cable can be installed immediately after drilling or293
incorporated into the well construction [Henninges et al., 2003], when re-freezing of the294
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hole would prevent access to logging devices [Hurtig et al., 1996]. Stotler et al. [2011] also295
make use of DTS to monitor permafrost and sub-permafrost temperature conditions.296
There has been renewed interest in carrying out thermal tracer tests, with passive297
temperature monitoring in a number of observation wells [Wagner et al., 2014]. This may298
be stimulated by the desire to directly study heat transport in the subsurface, or to use299
temperature as an easily monitored tracer for groundwater flow. The advantage of using300
DTS here is that multiple observation wells can be monitored simultaneously using the301
same instrument, either by using multiple cables attached to the same instrument, or by302
using one continuous cable that is looped inside each borehole [e.g. Read et al., 2013].303
Thermal tracer tests monitored using DTS have been carried out in a variety of settings304
including: a sedimentary aquifer [Macfarlane et al., 2002]; a fractured sandstone [Hawkins305
and Becker , 2012]; a fractured granite [Read et al., 2013]; and a shallow sedimentary306
aquifer [Hermans et al., 2015]. Recently, Bakker et al. [2015] carried out a thermal tracer307
test monitored with DTS without monitoring wells using fiber optic cable in the subsurface308
installed by direct push. In unconsolidated sediments, this approach allows thermal tracer309
tests to be monitored economically, in detail, and with minimal disturbance to the aquifer.310
4. DTS monitoring in Active mode (A-DTS)
In A-DTS, the cable [e.g. Read et al., 2014; Coleman et al., 2015; Hausner et al., 2015],311
borehole fluid [e.g. Leaf et al., 2012; Read et al., 2015], or surrounding rock formation312
is heated [Bakker et al., 2015]. The temperature response at some or all points during313
this process is then indicative of either the lithology, ambient groundwater flow, or in314
well flow, and depends closely on how the cable is deployed in the borehole and the315
nature of the thermal disturbance. These issues are the subject of this section. We have316
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categorized the various current A-DTS methods in Figure 3. These are thermal advection317
tests (Figure 3a), hybrid cable flow logging (Figure 3b), and heat pulse tests (Figure 3c).318
Figure 3 shows typical cable set-ups, heating locations, and data, for a well intersecting319
two fractures with additional layered heterogeneity. We assume that in this well ambient320
flow (fracture flow freely entering the uncased borehole) or pumping occurs (Figure 3a and321
b). Alternatively, there exists fracture flow in the formation but this flow is not entering322
the well (but flowing around it) due to the presence of a casing or by use of a liner (Figure323
3c). Each of these three principal methods is outlined in turn in the following sections.324
There are two principal strategies that have been trialled in hydrogeology to intro-325
ducing a thermal anomaly along the DTS cable in the subsurface to enable an A-DTS326
measurement. These are the physical injection of a fluid (usually water) with a contrasting327
temperature into the system, or the use of electrical heating techniques. In oil and gas ap-328
plications chemical pills have been used to create thermal anomalies in boreholes [Edwards329
et al., 2010] which is a method that has not been used in hydrogeological applications as330
far as we are aware.331
Injection of fluid with a contrasting temperature may be carried out at a single location332
and for a short period of time to create a discrete plume in the borehole [Leaf et al.,333
2012]. Alternatively, the injection may be carried out over a longer period to replace the334
fluid in a longer section of the borehole with fluid warmer or colder than the surrounding335
rock [Yamano and Goto, 2005; Read et al., 2013]. A second way to to create a thermal336
anomaly is to use electrical conductors within the DTS cable used for measurements to337
create a heating circuit, a separate heating cable or heating elements. Electrical heating338
techniques offer several advantages over injecting a fluid. The use of electrical heating339
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does not disturb ambient vertical flows of which there is a great risk during fluid injection340
into the borehole as described above. Electrical techniques make it also easier to quantify341
the amount of heat added to the system that can be readily calculated by integrating the342
input power over time.343
Electrical heating can be carried out along the entire or part of the length of the borehole344
or at point locations. Point heating can be achieved using small heating elements [Sellwood345
et al., 2015a; Read et al., 2015]. Uniform heating over lengths of borehole can be achieved346
with Joule heating along an electrical conductor [Kurth et al., 2013]. It is possible to347
directly heat the same cable used for DTS temperature monitoring by driving electrical348
currents through metallic armouring provided that an electrical connection is available to349
both ends of the cable [e.g., Sayde et al., 2015; Read et al., 2014], utilizing a composite350
cable design that incorporates parallel electrical conductors and optical fibers [Coleman351
et al., 2015], or by ad-hoc solutions that involve the manual wrapping of a passive DTS352
cable around a line source of heat [Liu et al., 2013; Seibertz et al., 2016]. Specifically353
designed and manufactured cables are typically required since standard cable jackets may354
not be designed to be safe at elevated voltage. Electrically insulated heating elements355
inside the same cable construction can be more economical, however, the geometry is not356
radially symmetric, and small differences in the separation between the heating element357
and optical fibers may cause temperature anomalies observable during heating. Even358
with a radially symmetric cable, there may still be longitudinal differences in heating359
[Cao et al., 2015]. For heated cable methods, the power output from the heated cable,360
P [W] can be calculated according to P = V I, in which V is the voltage drop along the361
cable [V], and I [A] is the electrical current. If constant power output through time is362
D R A F T October 31, 2016, 11:05am D R A F T
READ ET AL.: SUBSURFACE DTS X - 19
required, an active power controller is needed to supply constant total power (due to the363
temperature dependent resistance of conductors). The total power output can then be364
divided by the length of the heated section to obtain the heat generated per meter of365
cable. In down-hole applications, power inputs in the order of 5-20 W/m have been used366
successfully. However, the power input to be applied in any specific A-DTS application367
to generate the most informative dataset would vary with parameters such as borehole368
radius, potential for generating thermal convection inside the borehole, and the temporal369
and spatial resolution of the DTS measurement configuration.370
Since with A-DTS experiments a thermal anomaly (positive or negative) is introduced371
into the system, there is the possibility that water viscosity and density will be affected to372
the degree that convection is induced in the system. When this occurs such effects will need373
to be included in the interpretative framework of the A-DTS dataset. However, relatively374
simple calculations, for example using Rayleigh number analysis, can give already an375
indication of how important convection might be in a given system [Read et al., 2015].376
Hence, an A-DTS configuration might be designed to minimize the complications arising377
from such effects.378
4.1. Thermal Advection Tests
Thermal Advection Tests have been deployed to determine the location and significance379
of inflows and outflows from wells. In general, the principle is that the introduction of380
water with an anomalous temperature is tracked as it moves vertically in the well (Figure381
3a).382
The most common form of this method uses a point source to generate a heat pulse.383
In point heating experiments, the movement of a packet of either warmer or cooler water384
D R A F T October 31, 2016, 11:05am D R A F T
X - 20 READ ET AL.: SUBSURFACE DTS
in the well bore is tracked using DTS. If dispersion can reasonably be assumed to be385
symmetrical, then the movement of the temperature peak (or minimum) will be represen-386
tative of the cross-sectionally averaged velocity. The test is carried out with a fiber optic387
cable installed in the borehole fluid. The point heating approach is possible with either388
the injection of fluid [Leaf et al., 2012], or with point electrical heating [Sellwood et al.,389
2015a, b; Read et al., 2015]. For fluid injection, a thermally insulated hose is lowered down390
to the depth of interest and fluid pumped or allowed to drain in from the surface. Point391
electrical heating makes use of a submersible electric heater lowered down to the target392
depth. The aim of both approaches is then to create a short section of the borehole with393
anomalous temperature water that can then be tracked with DTS. Qualitative analysis394
of the DTS data obtained allow the local flow characteristics to be inferred, for example,395
the identification of vertical flow and the flow direction. Quantitative analysis involves396
calculation of vertical flow rates from the displacement of plume peaks between successive397
DTS temperature profiles.398
Figure 4 shows a point heating experiment carried out in a borehole penetrating a399
sparsely fractured aquifer, adapted from [Read et al., 2015], where it is referred to as400
the Thermal-Plume fiber Optic Tracking (T-POT) method. Here a point heater, at a401
depth of 68 m, generated a localized region of warmer water. Heating was switched402
off, and pumping at shallow depth simultaneously began, advecting the plume upwards.403
Assuming no significant heat loss from the bore to the formation, tracking the plume404
peak allowed the upward fluid velocity in the borehole to be calculated at 11.3 cm s−1.405
Through a series of controlled tests, Sellwood et al. [2015b] showed that thermal advection406
tests can be used to measure velocities down to below 0.06 cm s−1. If the vertical flux to407
D R A F T October 31, 2016, 11:05am D R A F T
READ ET AL.: SUBSURFACE DTS X - 21
be measured is high, then injecting fluid may be acceptable. However, if attempting to408
measure very low velocities, then fluid injections are likely to disturb the hydraulic head409
in the well and therefore the velocity itself. In such conditions, point electrical heating410
would be preferable. In an oil and gas reservoir setting Rahman et al. [2011] have carried411
out similar point heating experiments in well bores.412
A variant of this method is to heat or inject at a point continuously. The result is a413
front of warm or cold water that propagates vertically through the well. This has been414
applied mainly in deep [Yamano and Goto, 2005], and geothermal wells [Ikeda et al., 2000;415
Sakaguchi and Matsushima, 2000]. In such tests, the spacing of the front as it vertically416
propagates can be used to locate inflows and outflows. Additionally, the analytical solution417
presented by Yamano and Goto [2005] allows the temperature depth profile, following418
prolonged injection of water at the surface, to be inverted for flow.419
A third variant is to use a distributed, rather than point heat source. This was carried420
out by [Banks et al., 2014] using a heating cable and a wrapped high spatial resolution421
fiber optic cable separated by a fixed distance. Figure 5 shows DTS temperature data422
obtained prior to heating, during heating, and then after pumping was initiated at the423
top of the borehole. Once pumping was started, the inflowing fractures caused step-like424
reductions in fluid temperature. As suggested by Banks et al. [2014], it may be possible425
to derive a flow log based on the gradients of the temperature depth profile between426
fractures, and the size of the temperature reductions. While this method is not as simple427
as the point method for velocity estimation, it has the advantage that the entire borehole428
can be evaluated simultaneously.429
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X - 22 READ ET AL.: SUBSURFACE DTS
4.2. Hybrid cable flow logging
In-well vertical flow may also be monitored with hybrid cable flow logging. In this
method, heating occurs within the same cable as the optical fibers, and the cable is
deployed in a well in contact with flowing groundwater (Figure 3b). The principle is
that when the cable is heated with a constant power input over time, the temperature
inside the cable at steady state is a function of the velocity of the surrounding water.
Higher velocities reduce the cable temperature by more effectively thinning the thermal
boundary layer in the fluid around the cable. This method has also been used to estimate
wind speeds [Sayde et al., 2015] and seepage through dams [Aufleger et al., 2007]. Read
et al. [2014] show that for a typical armored cable with radial symmetry, the temperature
difference between the cable centre and water temperature ∆T is given by:
∆T =Q
2π
(1
hr2+
1
kclnr2r1
)(1)
where Q [W m−1], is the heat input to the cable, h [W m−2 K−1], is the heat transfer430
coefficient, r1 is the radius of the armoring, r2 is the total cable radius, and kc [W m−1431
K−1], is the thermal conductivity of the insulating material between r1 and r2. In practice,432
it is difficult to calculate h from first principles, however it can be readily found empirically433
by taking measurements at a number of known flow velocities during a field test [e.g. Read434
et al., 2014], or could be inferred from controlled lab experiments.435
Figure 6 shows values of ∆T collected in a pumped well from the dataset collected436
by Read et al. [2014], with a velocity profile obtained with an impeller flowmeter for437
comparison. The hybrid cable method identified step changes in flow from inflowing438
fractures that appeared as step reductions in the ∆T . The hybrid cable log also suffered439
from some artifacts due to cable deployment. Where the cable touched the wall, at around440
D R A F T October 31, 2016, 11:05am D R A F T
READ ET AL.: SUBSURFACE DTS X - 23
60 m depth, the ∆T value was higher. The presence of other objects inside the well, in this441
case 2 m spaced plastic centralizers to keep the heated cable in the center of the borehole,442
caused localized cooling thought to be due to the stimulation of vortices enhancing the443
heat transfer.444
The hybrid cable approach has the advantage that it is possible to monitor time varying445
velocity and flow changes more readily than with point thermal advection tests. However,446
since the response is sensitive to anything altering the efficiency of heat transfer from447
the cable, effects due to varying cable centralization or other instrumentation in the well448
disturbing the flow may be apparent. It is also likely to require empirical calibration in449
the field and each well prior to use.450
4.3. Heat pulse tests
Apparent thermal properties along a wellbore can be characterized using DTS heat451
pulse tests. This method is based on applying a constant power output over the entire452
deployed cable length for a period of time and monitoring the heating and/or cooling453
phases using DTS [Coleman et al., 2015; Hausner et al., 2015; Seibertz et al., 2016]. The454
measured thermal response is dependent on both conductive and advective components of455
heat transfer. Through careful analysis information regarding both lithological variations456
and natural gradient flow distributions can be gathered.457
In heat pulse tests, heating or cooling data may be used. An analytical solution exists
for constant heat injection in a homogeneous, radially symmetric porous medium. This
is given by [Shen and Beck , 1986]:
ka =Q
4π· ln (t2/t1)
T2 − T1(2)
D R A F T October 31, 2016, 11:05am D R A F T
X - 24 READ ET AL.: SUBSURFACE DTS
where ka [W m−1 K−1] is the apparent thermal conductivity of the formation, Q [W458
m−1] is again electrical power input, and T1 and T2 are the temperatures at times t1 and459
t2 respectively. Heat Pulse Tests can be conducted using either separate heating and fiber460
optic cables or with a composite cable that incorporates both resistance heating wires461
and optical fibers. Early installations [e.g., Freifeld et al., 2008], utilized separate cables462
for heating and temperature measurements in which the distance between these cables463
is variable with depth. Temperature measurements during heating are highly dependent464
on the distance between the heating wires and optical fibers; thus, cooling data were465
exclusively used for data analysis where distance variations had much less of an effect.466
The application of composite cables has allowed for both heating and cooling data to467
be analyzed [Coleman et al., 2015; Hausner et al., 2015]; however, cable materials and468
geometry as well as installation geometry still play a major role in the measured response469
and is an area of ongoing research. For example, variable cable coupling between the rock470
formation and wellbore fluids can negatively impact the measurement response. Care471
must be taken when applying equation 2 for composite cables. The initial temperature472
increase will be due to the thermal properties of the cable, so late time data must be473
used. Detailed numerical modelling can be used to assess in more details the relevance474
of these various complicating factors for the interpretation of A-DTS data sets [Hausner475
et al., 2015].476
Since heat pulse tests measure apparent thermal properties, specific information regard-477
ing the field installation is required if isolating the conductive and advective components478
of heat transfer is desired. Freifeld et al. [2008] deployed cable inside a 535 m deep bore-479
hole through volcanic strata in a permafrost environment, and used a 1-D radial model to480
D R A F T October 31, 2016, 11:05am D R A F T
READ ET AL.: SUBSURFACE DTS X - 25
invert cooling data from a heat pulse test to determine a thermal conductivity profile. In481
this case both natural gradient flow and vertical wellbore flow variations were assumed to482
be negligible in order to calculate thermal conductivity profiles. If the thermal properties483
of the rock strata are known there is significant potential for using apparent thermal data484
to quantify flow [Coleman et al., 2015]. Figure 7 highlights the modeled effect of flow485
on Heat Pulse temperature data in comparison with field data collected in a dolostone486
aquifer. Differentiating natural gradient groundwater flow from lithological variations re-487
mains challenging. To determine lithological changes, groundwater flow must be known488
or assumed; whereas, to isolate groundwater flow thermal properties of the rock must489
be known a priori. In either case the borehole should be cased or sealed using a flexi-490
ble borehole liner to eliminate cross-connected wellbore flow that would cause significant491
measurement bias.492
5. Future Developments
Both the hybrid cable and heat pulse methods are at a relatively early stage of devel-493
opment in hydrogeological applications. Whilst in oil and gas studies similar methods494
are already more widely applied [Bolognini and Hartog , 2013] they are mostly used as495
qualitative monitoring solutions. The main development to be looking forward to is to496
make down-hole DTS methods into tools from which quantitative data can be gathered497
on hydrogeological conditions outside the borehole. Numerical modeling can be expected498
to be an important route in this process as they can consider the complex geometries as-499
sociated with borehole installations and its impact on the strongly coupled fluid and heat500
flow processes associated with (A-)DTS experiments. Such simulations should be used to501
assess and quantitatively interpret DTS data sets for hydrogeologically relevant param-502
D R A F T October 31, 2016, 11:05am D R A F T
X - 26 READ ET AL.: SUBSURFACE DTS
eters (e.g. formation groundwater flow). Moreover, prior to field installations they can503
serve to trial the large range of options for deployment (e.g., configuration of the cable in504
the system, optimum A-DTS heat input) that many current researchers are so creatively505
exploring. This is critical since, as we have shown in this review, all of the choices as-506
sociated with the design of a down-hole DTS deployment will impact the effectiveness of507
DTS in obtaining valid and valuable data.508
While there are many literature examples of subsurface DTS application, there is plenty509
of opportunity for future development of the technology in groundwater studies as instru-510
ment and cable performance improve. Further work is also needed to assess the sensitivity511
of active methods under different conditions to both in well vertical flow and natural gra-512
dient flow, and turn them into readily deployable monitoring solutions providing quanti-513
tative results. In order to do this auxiliary data are needed to complement DTS data sets.514
Spatially distributed data like DTS are likely to originate from other hydrogeophysical515
methods which are still actively developed [Binley et al., 2015]. DTS can be combined516
with such methods like it is with, for example, with Distributed Acoustic Sensing which517
is another optical-fibre sensing method [Noni et al., 2011; Mondanos et al., 2015].518
6. Conclusion
Our overview has highlighted examples where DTS has been used in studies of aquifer519
heterogeneity, groundwater flow, subsurface heat transport, and geothermal and CO2520
sequestration. Such deployments are typically either passive or active. In passive applica-521
tions, DTS can provide near real time temperature monitoring, simultaneously, potentially522
in multiple deep wells at high spatial resolution. This allows dynamic processes to be mon-523
itored in detail, and long data sets to be efficiently collected. Active DTS methods aim524
D R A F T October 31, 2016, 11:05am D R A F T
READ ET AL.: SUBSURFACE DTS X - 27
to primarily measure in-well flow (thermal advection tests and hybrid cable flow mea-525
surements), and thermal properties and natural gradient groundwater flow (heat pulse526
tests). While there are some similarities between the active methods, it is important to527
note the differences in the sensitivities of each method and the underlying physics. When528
considering undertaking field or laboratory experiments active and passive methods may529
easily be combined to efficiently obtain different but complementary data. For instance,530
after heating a cable for a long duration, it may be possible after hybrid cable flow logging531
to monitor the return of the slightly warmed borehole to background temperature. The532
first measurements would give flow in the well, while the second may yield information533
on aquifer heterogeneity and thermal properties.534
When to use DTS in a borehole setting requires careful consideration. If there is a535
passive process to be monitored with either little time or depth variability, then a log536
obtained with a high temperature resolution temperature probe, or a time series from537
a data logger at a particular depth might yield equivalent or better datasets with less538
effort. However, DTS is proving invaluable in applications where there is significant time539
and depth variability. Here, the required effort in terms of installation of power supply,540
calibration baths, and the post-processing of the data should far be outweighed by the541
benefits to hydrogeological research provided by the wealth of additional detail in spatio-542
temporal temperature dynamics recorded by DTS .543
Acknowledgments. We thank the editorial team, Associate Editor Andrew Binley,544
Reviewer Nick van de Giesen, and two anonymous Reviewers for their time and energy545
spend to provide constructive feedback on earlier versions of this manuscript. Fund-546
D R A F T October 31, 2016, 11:05am D R A F T
X - 28 READ ET AL.: SUBSURFACE DTS
ing for this work was provided by a Natural Environment Research Council (NERC)547
studentship (NE/J500069/1) to Tom Read. Some of the data come from the national548
network of hydrogeological sites H+. The ANR project CRITEX ANR-11-EQPX-0011549
provided partial funding for Olivier Bour and Tanguy Le Borgne. John Selkers contri-550
butions to this work were made possible by grants from the US government via NASA551
award NNX12AP58G and the Center for Transformative Environmental Monitoring Pro-552
grams (CTEMPs) funded by the National Science Foundation, award EAR 0930061. Data553
underlying this study can be obtained upon request from the corresponding author (vic-554
[email protected]).555
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Figure 1. Schematic of DTS principles based upon Raman backscatter detection. In this
cartoon a fibre-optic cable is deployed in a duplexed single-ended set up. A double-ended mea-
surement would be possible by additionally connecting the red optical fiber to the instrument,
and alternating measurements between the two instrument channels
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X - 40 READ ET AL.: SUBSURFACE DTS
e)b) c) d)a)
De
pth
[m
]
Temperatures at point
locations every 0.1 m;
borehole camera record
showing location of fracture
DTS spatial sampling: 1.01 m
spatial resolution: 2 m
DTS spatial sampling: 0.3 m
spatial resolution: 0.125 m
DTS spatial sampling: 1.01 m
e!ective spatial resolution:
0.3 m (wrapped cable)
DTS spatial sampling: 0.3 m
e!ective spatial resolution:
0.03 m (wrapped cable)
Figure 2. Section of a temperature-depth profile collected in an open well at the Ploemeur
research site, France, showing a) optical televiewer and temperature depth-profile measured with
a high precision temperature probe measuring every 0.1 m, b) 2 m spatial resolution (1 m spatial
sampling) instrument with a standard cable, c) 0.3 m spatial resolution (0.125 m spatial sampling)
instrument with a standard cable, d) 2 m spatial resolution (1 m spatial sampling) instrument
and wrapped cable, and e) 0.3 m spatial resolution (0.125 m spatial sampling) instrument and
wrapped cable. Each DTS temperature-depth profile is a 10 minute time average collected on
the same day. Grey curves (one highlighted in red) show the Gaussian spatial weighting function
for each DTS-reported temperature measurement along the cable (marked with x’s in the plots),
calculated using the method given in Selker et al. [2014]
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READ ET AL.: SUBSURFACE DTS X - 41
a)
Temperature
De
pth
Heat Pulse TestThermal Advect ion Test Hybrid Cable Flow Logging
t= 0
t= β
t= 2β
t= 3β
t= 4β
t= 5β
t= 6β
Δz
2Δz
ΔT
heat ing phase
cooling phase
Temperature Temperature
b) c)
Power cable (non-heated)
DTS cable
Fluid flux towards and in the boreohle.In case of (c) fluid flow is around the cased borehole
Heating cable/element
Combined (’hybrid’) heating/DTS cable
Figure 3. Schematic of three Active-DTS methods in a well intersecting two discrete transmis-
sive zones (e.g. fractured intervals) and varying lithology. Right-hand side panels in (a-c) show
the diagnostic data to be observed for each test. a) Thermal Advection Test with temperature
depth profiles after point injection or point heating resulting in a quantification of flow in the
borehole driven by inflow from discrete fractures, b) Hybrid Cable Flowmetry in which in-well
flow is quantified, and c) a Heat Pulse Test with temperature data from the heating and cooling
phase as observed outside the borehole either by using a liner [e.g. Leaf et al., 2012] or by instal-
lation of DTS cables in a borehole casing grout, or via direct-push techniques (not shown) in the
absence of a borehole [Bakker et al., 2015]. Heating- and cooling data can be used to infer appar-
ent thermal properties of the formation that can be related to both heat-advection (groundwater
flow) and heat-conduction in the formation. See the text on each method for further detail.
D R A F T October 31, 2016, 11:05am D R A F T
X - 42 READ ET AL.: SUBSURFACE DTS
T
s
t
Figure 4. Electrical point heating experiment carried out in a well with ambient vertical
flow, then stimulated by pumping (adapted from Read et al. [2015]), showing a) DTS measured
temperature showing the distance (s) travelled (upward in this case) by the thermal anomaly
introduced at the point heating element within a certain time interval (∆t). The water velocity
in the well is given by s/∆t. b) The water level in the well during the test. Pumping was initiated
at 12:33D R A F T October 31, 2016, 11:05am D R A F T
READ ET AL.: SUBSURFACE DTS X - 43
Figure 5. Schematic response after heating an open borehole with separate heating cable in
both ambient conditions and then during pumping, taken from Banks et al. [2014]
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X - 44 READ ET AL.: SUBSURFACE DTS
raw DTS data
processed
DTS data
Velocity [m s-1]
impeller velocity
measurement
ΔT [oC]
De
pth
[m
]
Figure 6. Comparison of ∆T profile and velocity point measurements with an impeller
flowmeter during a pumping test, adapted from Read et al. [2014]. The ’processed’ ∆T profile is
the result of a moving median filter applied to the original data in order remove the local effect
of the cable centralizers used in the deployment
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READ ET AL.: SUBSURFACE DTS X - 45
Figure 7. Field data from a borehole heat pulse test in a borehole with strongly varying
groundwater influxes [Coleman et al., 2015]. Field data show a wide range of responses and
models have been used to provide an initial indication over which range the these fluxes would
vary to understand the observed varaibility is DTS measured responses and hence the sensitivity
of the measurements to the groundwater influx.
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